1. Field of the Invention
The present invention relates to a technology for accurately detecting a grammage of a recording material.
2. Description of the Related Art
Up to now, some image forming apparatuses such as a copier and a printer are provided with a sensor configured to determine a type of a recording material inside thereof. In these apparatuses, the type of the recording material is automatically determined, and transfer conditions (for example, a transfer voltage and a conveyance speed of the recording material at the time of transfer) and fixing conditions (for example, a fixing temperature and a conveyance speed of the recording material at the time of fixing) are controlled in accordance with a determination result.
Japanese Patent Laid-Open No. 2010-18433 describes an image forming apparatus provided with a grammage detection sensor configured to detect a grammage of a recording material by irradiating the recording material with an ultrasonic wave and receiving the ultrasonic wave that has attenuated via the recording material. In this image forming apparatus, image forming conditions such as transfer conditions and fixing conditions are controlled in accordance with the grammage of the recording material detected by the sensor. In addition, it is known that, in the above-described grammage detection sensor using the ultrasonic wave, the detection result varies depending on a surrounding environment where the sensor is installed (for example, a barometric pressure or a temperature). For that reason, according to Japanese Patent Laid-Open No. 2010-18433, by comparing a result of the reception of the ultrasonic wave in a state where the recording material does not exist with a result of the reception of the ultrasonic wave in a state where the recording material exists, an influence on the detection result by the change in the surrounding environment is suppressed.
However, the surrounding environment of the grammage detection sensor changes even in mid-course of image formation on plural sheets of the recording materials. In particular, in a case where the images are continuously formed on both surfaces of the plural sheets of the recording materials, the recording material that has once passed through a fixing device to be warmed up passes through in the vicinity of the sensor again, and therefore a temperature in the surrounding of the sensor is increased. When the temperature in the surrounding of the sensor is increased, the result of the ultrasonic wave reception is changed, and therefore the grammage of the recording material may be wrongly detected in some cases.